library(MASS)
a1 = c(1,1,-1)
a2 = c(1,-1,1)
a3 = c(2,0,0)
b = c(2,0,0)
A = matrix(c(a1,a2,a3), nrow=3, ncol=3)
m = Null(A) # null space of matrix A 
# a special solution is obtained by using generalized inverse of matrix
# see https://en.wikipedia.org/wiki/Moore%E2%80%93Penrose_inverse#Obtaining_all_solutions_of_a_linear_system
x_s = ginv(A) %*% b 
